Crystalline silica is found in nature in three distinct structures, i.e., quartz, tridymite and cristobalite, as identified by x-ray diffraction analysis with each crystalline structure having polymorphic forms stable in different temperature ranges. In each such crystalline form of silica a silicon atom is bonded to four oxygen atoms in tetrahedral coordination. Each form represents a distinct arrangement of silicon-oxygen tetrahedra into a three-dimensional crystalline network. Generally, crystallization temperature determines which structure is formed. Quartz is formed by crystallizing silica at below about 867.degree. C.; tridymite is formed at crystallization temperature of about 867.degree. C. to about 1470.degree. C.; and cristobalite is formed at temperatures above about 1470.degree. C. Cristobalite exists in two polymorphic forms, i.e., low cristobalite, also referred to as alpha cristobalite, and high cristobalite, also referred to as beta cristobalite. High cristobalite, the stable form of crystalline silica at temperatures above about 1470.degree. C., ordinarily converts to low cristobalite when cooled through the temperature range of about 200.degree.-275.degree. C.
High cristobalite is one of the best refractories available for use in ovens and furnaces where the temperatures encountered are well above 1,000.degree. C. and up to 1,450.degree. C., due to its low coefficient of thermal expansion.
Silica brick is the refractory commercially available and it usually contains a mixture of low cristobalite, other low temperature forms of SiO.sub.2 including quartz and tridymite, residual glass and some anorthite. When the silica brick refractory is in place as a lining material for a furnace, and is subjected to temperatures up to 1,500.degree. C., substantially all of the other forms of silica are converted over a period of time to the high cristobalite phase. However, when the furnace is shut down for repairs or other reasons and the high cristobalite-containing silica brick is cooled to ambient temperatures, an inversion of the high cristobalite to low cristobalite occurs when the brick is cooled below 200.degree.-275.degree. C. This inversion to low cristobalite produces a change in the volume and symmetry of the high cristobalite, which change results in spalling and cracking of the silica brick, rendering it unusable as a furnace lining. Such bricks must be replaced before the furnace can be fired up again.
The inversion of high cristobalite to the low form upon cooling to below about 200.degree. C. is recognized in U.S. Pat. No. 4,073,655.
Disclosed therein is the teaching that CaO is essential to produce a stabilized high cristobalite from the devitrification of a glass.